docs: def restricted_to_files(self,
                              filenames: List[str]
                              ) -> 'Spectra':
          """
          Returns a variant of this spectra that only contains entries for
          lines that appear in any of the files whose name appear in the
          given list.
          """
          tally_passing = \
              {fn: entries for (fn, entries) in self.__tally_passing.items() \
               if fn in filenames}
          tally_failing = \
              {fn: entries for (fn, entries) in self.__tally_failing.items() \
               if fn in filenames}
          return Spectra(self.__num_passing,
                         self.__num_failing,
                         tally_passing,
                         tally_failing)